Vehicle radiators have two manifold tanks that interconnect a plurality of generally parallel flow tubes, feeding coolant through the tubes in one or several passes. In the case of multi-pass flow, the interiors of one or more of the tanks contain solid baffles which divide the tanks up into two or more sections, thereby dividing the flow tubes up into two or more passes. Radiators may also be mounted in a cross flow or down flow orientation. In a cross flow orientation, the tanks are generally vertical, one on either side of the radiator, with horizontal flow tubes. A down flow radiator is turned ninety degrees, with one tank horizontal at the top, one horizontal at the bottom, and generally vertical flow tubes. With either orientation, a removably capped filler neck opens through one end of one tank (either a side tank or a top tank) to allow the radiator to be filled with coolant. Fluid fill is infrequent, occurring either during vehicle assembly or later, during vehicle service. In either case, the coolant displaces air in the radiator, which needs to be vented to assure a solid and complete fluid fill. In a single pass radiator, with no baffles dividing up the interior of the tank, air bubbles in the fluid will naturally float and flow upward without interference toward the highest open point in the radiator, which is the open filler neck, where they can ultimately vent to atmosphere. With a multi pass design however, especially a two pass radiator with a down flow orientation, a unique venting problem is presented. A flow dividing baffle divides the center of the top tank into two sections, a first section opposite the filler neck, to which the filler neck cannot directly open, and a second section into which the filler neck does open. During coolant fill, air floating upwardly into the top tank's first section will be blocked by the baffle from reaching the second section or the open filler neck, which would jeopardize an adequate fluid fill.
U.S. Pat. No. 5,762,130 issued Jun. 9, 1998 to Uibel, et al. and co-assigned to the assignee of the subject application, discloses a manually operated vent valve near the upper tank baffle which can be selectively accessed and opened from outside the tank to vent trapped air from one side of the baffle to the other. This would be done only infrequently, such as just after coolant fill. It requires a deliberate operation on the part of the assembly or service worker, which takes time. Once the valve is reclosed, there is no provision for venting any air that might be trapped later, from whatever source. Furthermore, the manual valve does pierce the tank wall, creating a potential leak path out. There is also a smaller possibility that air could invade the cooling system during normal operation, not just during coolant fill, as the result of a system leak. This would become trapped and remain trapped on the downside of the baffle, barring a deliberate opening of the valve, which would be very unlikely, as there is no visual indication of such trapped air.